Low capacitance power supply



April 4, 1961 J. H. REAvr-:s 2,978,658

LOW CAPACITANCE POWER SUPPLY Original Filed May 9, 1955 2 Sheets-Sheet 2 III- 415 vL/oov-I f- INVENTOR HTTUR/VEYS United States Patent O '2,978,65 8 LOW CAPACITANCE POWER SUPPLY May 9, 195s, ser.y N0. 507,186. kthis application Nov. 29, 1957, Ser. No.

Original application Divided and 699,873

6 Claims. (Cl. 335-69) This invention relates to electronic amplifiers and particularly contemplates an improved power supply for direct-coupled amplifier circuits.

The present invention is a division of applicants copending application Serial No. 507,186, filed on May 9, 1955, `for a Direct-Coupled Amplifier Construction.

Some direct-coupled amplifier circuits require the use of a direct-current power source which can neither be grounded nor yby-passed to ground if proper operation of the circuit is to be obtained. Inv such instances a conventionaly power supply cannot be used because of the large inherent shunting capacity of a conventional type of power transformer. Although batteries would be suitable in such application ythey suffer the disadvantages of short life, and unadjustability of voltages.

Conventional power transformers such as are employed in typical power supplies are designed for maximum power transfer eliiciency, and because of the required close proximity between the windings and core in order to secure maximum coupling, such construction results in characteristically high capacitance between the windings and between the windings and core. When it is attempted to employ a conventional power supply in a circuit in which the power supply cannot be grounded or lbypassed to ground, the effect of the capacitance is to shunt signals toground. Moreover, even should a conventional power supply be employed in connection with such described type of circuit, signal losses would occur for all but the lowest frequencies. The present invention overcomes such limitations through the use of a power 'supply circuit which employs a specially constructed power transformer in which the secondary capacitance has been reduced to an extremely low value. Because of such feature it becomes possible to construct various embodiments of a novel direct-coupled amplifier employing the power supply according to this invention.

A low capacity transformer construction for filament supply use is disclosed in U.S. Patent No. 2,314,083 by I. L. Finch, but so far as is known no attempt has heretofore been made to employ a low-capacitance power supply for plate and bias voltage supply.

It is therefore an immediate object of, this invention to provide a power supply, particularly adaptable to direct-coupled amplifier construction, which possesses a low shunt capacity and can therefore be used in circuits requiring the power supply to operate at a (signal) varying potential level with respect to ground.

A further object of this invention is to provide a power supply employing a special transformer construction in which the secondary lwindings are capacitively separated from the core and from the primary winding.

It is an additional object of this invention to provide a power supply which is particularly adaptable `for use in direct-coupled circuit applications where neither terminal of the power supply can be grounded, or ybypassed to ground. n

A still further object of this invention is to provide a Y2,978,658 Patented Apr. 4, 1961 ICC 2 power supply in which the rectifying and filtering components can be physically mounted remote from those circuit elements which are at ground potential, and in proximity to the secondary winding of the transformer so vthat the shunt capacitance of the entire secondary circuit is kept at a minimum value.

An additional object of this invention is to provide various embodiments of direct-coupled amplifier circuits in which the voltage requirements are obtainable from a simple unitary type of A.C. operated power supply.

Other uses and advantagesA of the invention will be apparent upon reference to the specification and drawings in which: n i

Fig. 1A is an isometric view showing a structural embodiment of the present invention;

Fig. 1B is an isometric view showing the transformer construction in greater detail;

Fig. 2 is a circuit diagram of an electronically regulated power supply circuit used vin connection with the present invention;

Figs. 3A through 3C show various circuit modifications employing the power supply according to the present invention for interstagecoupling and as a pentode-cathode follower circuit-having-good high frequency response;

Fig, 4A shows the present invention applied to a voltage amplifier circuit suitable -for driving a capacitive load;

Fig. 4B shows a two-tube cathode follower circuit employing the features ofthe present invention suitable for driving a capacitive load, and

Fig. 4C shows a modification of Fig. a pentode.

The above objects are achieved according to the teachings of the present invention by employing a specially constructed power transformer in which the secondary winding is capacitively separated from the core, leaving as much air gap as the geometry of the construction permits. The effect of such preferred construction is to lower the capacitanceA of the secondary winding of the transformer in respect to the core and primary windings, respectively.

Since the secondary winding forms an element of the signal circuits in the particular amplifier application in which the power supply is used, any shunt capacity effect is accordingly minimized. The construction of this invention also contemplates the mounting of the rectifying and filtering components of the transformer secondary circuit relatively far from objects which are at ground potential. These components are further mounted in a novel fashion adjacent to the transformer on an insulated chassis which, for all practical purposes, is an integral part of the form which contains the secondary winding. Such construction results in a very low value to the stray or shunting capacitance of the entire secondary circuit.

Fig. lA shows a practical embodiment of a power supply employing the features of this invention. As shown in Fig. 1A the power supply includes a power transformer having a laminated core 1 of a size and configuration commensurate with the voltage and power requirements. The primary winding consists of a coil 2 having two sections each mounted on one leg of the transformer as shown in Fig. 1B. Only one section is shown in Fig. 1A. The coils are series connected and have external leads 3-3 adapted to be connected to a suitable 60-cycle A.C. source. As indicated, the primary windings are each mounted on a respective spool 4 which is made of a suitable insulator such as Plexiglas, Lucite, etc., and are mounted on each leg of the core.

In order to reduce the capacitance elect of the secondary circuit of the vpower supply to a minimum, the sec- 4A employing ondary winding is physically mounted on a large spool 6, which, as shown, has a re-entrant portion which passes through the throat of the transformer core. The spool for the secondary winding is also made of `.an insulating material, such as Plexiglas, and is arranged relative to the transformer core as indicated in Fig. l, so that the secondary winding 5 is separated from the core as well as from the primary windings bythe maximum amount of air gap which the geometry of the construction permits. A s is apparent from Figs. lA and 1B, only 'the re-entrant portion of the spool need be in proximity to the core and primary winding in order to obtain magnetic coupling between the windings necessary for transformer action. It follows that any capacitive effects b etween the secondary and primary winding are substantially limited to the region involving the re-entrant portion of the secondary.

The physical characteristics of the various components employed in a typical power transformer construction according to the present invention may be tabulated as follows:

Transformer core dimension Cross-section x 1% inches, Outside dimension 3 x 3 x 1% inches.

Primary windings Y Number of turns 2 coils, 500 turns each. Wire size No. 28.

Spool dimensions Cross-section 1%@ X 11%6 inches. Length 1%6 inches.

Secondary windings Number of turns 5200 center-tapped. Wire size No. 34.

Spool dimensions Cross-section 2% x 1% inches. Length 1% inch.

The core of the transformer is constructed of transformer steel, the lamination thickness being approximate- 1y 0.017 inch, a value suitable for 60-cycle operation and which value may be decreased for higher frequencies of operation. Enameled wire is preferred for all windings because of its compactness, and the primary consists of two aiding series-connected coils mounted on opposite legs of the square periphery core 1. Such construction reduces magnetic leakage while permitting a maximum amount of clearance for the secondary winding. A transformer construction in accordance with the above specifications is capable of delivering approximately 30 milliarnperes at 160 volts while manifesting only 18 micromicrofarads capacitance to ground. It is to be noted that the shunt capacitance of a conventional comparable power supply generally is approximately 700 micromicrofarads.

The above-featured power transformer construction further permits the mounting of therectifying, filtering, and voltage regulating'elements of the power supply in a novel manner, a construction which results in a very low value to the stray or shunting capacitance of the entire secondary circuit.

The physical arrangement of the rectifying, filtering, and voltage regulating components of a typical power supply according to this invention is shown in Fig. 1A.

may be the panel as illustrated in Fig. 1 may readily be identied, and their circuit relationship established in connection with the diagram of Fig. 2.

Specifically, as the circuit diagram indicates, a rectifier V-1 of the 6X4 variety, for example, is mounted in a suitable socket 14 secured directly to the panel 11. The tubes V-2, V-3, and V-4, which are of the type indicated in Fig. 2, comprise the voltage-regulator circuit of the power supply and are mounted in suitable sockets 15, 16, and 17 in convenient relationship to the rectier tube on the panel 11. T he three-section filter condenser C-18 and the voltage control potentiometer 19 arev also secured to the mounting panel 11.

Returning to the description of the power transformer, it will be noted from Fig. 2 that the secondary winding 5 is designed to provide a plurality of output voltages. The three heater windings 5a, 5b, and 5c in the transformer mounted on top or underneath the high voltage secondary winding. The remainder ofthe circuit elements comprising the power supply are arranged on the lower surface of the mounting panel 11 in accordance with the wiring diagram of Fig. 2. Suitable output ter- "y minals '20 are provided on the mounting panel 11.

The electrical configuration of the typical regulated power supply circuit employed in the present invention is conventional except that it is ungrounded and will therefore not be described in detail. A general description of an equivalent grounded supply is given on pages 378379-of Electronics Experimental Techniques by Elmore and Sands.

The isolation of the secondary winding causes the efficiency of the transformer to be lowered somewhat, partly because of the decrease in the resulting magnetic coupling, and partly because of the increased circumference of the windings. Although for the low-power applications contemplated in the subsequent description,

poor power efficiency is not in itself serious, the accom- As indicated, all of such elements are mounted on an insulated platform comprising a mounting panel 11 provided with a pair of transverse mounting flanges 12-12. The material of the panel and lianges may be any convenient insulator, such' as Plexiglas,Lucite, etc. The power transformer described includes a plurality of mounting studs 13 to which anges are'secure'd. All of the electronic components comprising the rectifying, ltering and voltage regulating elements are compactly mounted on the panel 11 as indicated.`

Fig. 2 shows the complete circuit diagram for the power supply, the physical embodiment of which'is illustrated in Fig. 1. The various components mounted on panying increase in percentage voltage drop under load (regulation) is important. However, by employing electronic stabilization in the power supply circuit, as shown in Fig. 2, good regulation of the D.C. output voltage can be achieved in spite of the relatively poor regulation of the transformer.

The power supply construction according to the specication described provides full wave rectification and with the following regulation characteristics:

The above-indicated shunt capacity value was measured with the power supply resting in an upright position on a grounded metal plate, the primary winding being grounded. The capacitance of the power supply constructed in accordance with the described specifications, measured only 18 upf. (mmfd.) as indicated. By comparison, the shunt capacity of a typical conventional supply for the same power measured approximately 700 auf. Much of this difference is attributable to the special transformer construction described.

In order to determine the relative merits of the power supplies constructed in accordance with the teachings of this invention, a suitable figure of merit was assumed to be the ratio of the maximum power output in milliwatts to the total shunt capacitance in puf.

Several power supplies employing the special lowcapacitance type of construction described were constructed for various current and volta-ge outputs as is indicated in Figs. 4A and 4B which show various exemplary voltage requirements. Y f The usefulness of the low-capacitance type of power supply can be demonstrated by several examples of novel A novel direct-coupled amplifier construction employ-k ing the power supply of the present invention is shown in Figs. 3A and 3B, the power supply being used in two different ways to provide direct interstage coupling. The output terminals A and B of the power supply circuit 'shown in Fig. 2 are similarly identified in the various embodiments illustrated in Figs. 3A-3C and 4A-4C,

Referring to Fig. 3A, the` power supply employedis ungrounded and has avery low secondary capacitance, therefore, signal attenuation due to the referred to shunt capacity effects is extremely small even for very high signal frequencies. In the particular embodiment of the invention shown in Fig. 3A, the power supply is not required to supply any current.

The modification of Fig. 3B shows the power supply providing the plate current for stage V- while serving as the coupling means between stages. In this modification, the magnitude of the plate voltage is not critical especially if a pentode-type tube is used for the amplifier V-30 in which case the screen would be held at a constant potential with respect to cathode and ground. In Fig. 3B there is also symbolically illustrated in dotted lines the capacitive shunting effect (C) imposed on the circuit when a conventional power supply is used.

The advantages ofan amplifier construction embodying the principles of the present invention can best be appreciated by comparison with a conventional direct-coupled amplifier such as is described on pages 529-531 (see Fig. 12.44) of the Radiotron Designers Handbook. While, in such type of construction, the shunt-capacity effects of the power supply is not important because it is already grounded, there exists the disadvantages of (l) signal attenuation by the voltage divider action across the resistors R-Z and R-3, Iand (2) the voltage divider circuit has to be capacitively compensated by the inclusion of a capacitor across the resistor R-Z in order to compensate for the shunting effect occasioned by the tubes input across the resistor R-3. In conventional directcoupled amplifier circuits such as are represented in the referred-to text such compensation is critical and sometimes unsatisfactory due to variations in the input impedance to the tube. Moreover, as compared to a conventional direct-coupled amplifier circuit construction, such as described in Fig. 12.44 of the referred-to handbook, the circuit shown in Fig. 3B is relatively insensitive to variations in the value of the supply voltage.

The power supply is shown in Fig. 3C as being employed to provide screen-grid potential in a pentodecathode follower circuit, As shown, terminal A of the described power supply is connected to the screen grid of a pentode V-32 while termin-al B is connected to the tube cathode. The output is obtained across resistor R-3.

Two direct-coupled circuits in which the novel power supply has been found to be particularly useful are illustrated in Figs. 4A and 4B. The two-tube amplifier illustrated in Fig. 4A employs a pair of tubes V-40, V-41 of the 6AF4 type. Terminal B of the power supply is shown connected to the cathode of tube V-40 While terminal A is connected through a resistor R-40 to the plate of tube V-41. The input signal is applied across the grid of tube V41 and the output is obtained across the cathodes of tubes V-40 and V-41 respectively as indicated.

The two-tube cathode follower circuit employing the power supply according to the present invention is shown in Fig. 4B. In this circuit the tubes V-42, tl-43 may be of the 6CL6 type and the power supply is connected as indicated to the plate of V-43 and to the cathode of V42, respectively. The input signal is applied to the grid of V-42 and the output is obtained across the circuit includ# ing the power supply and tube V-43 in series.

With a load that is predominantly capacitive and a signal that has a high duty factor, either of the circuits shown in Figs. 4A and 4B gives better high-frequency response fora given average plate current than a conven- Itional single-tube amplifier or cathode follower. While circuits of this general type, employing two series-connected tubes driven in opposite phase, are described in U.S. Patent No. 2,358,428, issued to E. L. IC. White, and by P. G. Sulzer` in an article entitled, Survey of Audio `Frequency Power-Amplifier Circuits, appearing in i bootstrap type.

Audio Engineering, vol. 35, No. 5, May 1951, the particular rtwo-tube 'direct-coupled circuits described in connection with Fig. 4A and Fig. 4B are believed to be novel because of the singular relationship between the circuit and the power supply employed.

The amplifier circuit of Fig. 4A has, in addition te low output impedance, good linearity'and a voltage gain very nearly equal to the amplification factor of stage V-41. An improved version of this circuit is obtained by using a pentode in place of a triode for'the tube V-40 as shown in Fig. 4C. In such case, the power supply is not needed in the position shown in Fig. 4A but instead is connected as in Fig. 3C so as to supply the screen-grid potential in the manner illustrated in Fig. 4C. The amplifying circuit o'r Fig. 4A can be modified by adding a capacitor 41 from the plate of V-41 to ground thus making a novel linear sawtooth generator of the familiar The two-tube cathode follower circuit of Fig. 4B has higher input impedance, better linearity, nearer unity gain, and lower output impedance than a conventional single turbe cathode follower. As an example of its high efficiency when driving a capacitive load, this circuit with the component values shown, will supply peak charge and discharge current of ma. with an average plate circuit current as low as 15 ma. A conventional single-tube circuit giving comparable square-wave output with the same load capacitance requires an average plate current of approximately ma.

The above exemplary embodiments typify instances in which direct-coupled circuits must be supplied with a D.C. source which cannot be grounded or bypassed to ground. By combining the novel low-capacitance power supply with such direct-coupled circuits in accordance with the teachings of this invention it becomes possible to construct such circuits for convenient A.C. operation and thereby eliminate the need for batteries.

While a preferred embodiment of the invention has been disclosed and described in the accompanying drawings, it will be apparent that various modifications and embodiments of the singular features of the invention described would readily follow from the present disclosure. For example, in the construction of the power transformer, the primary windings could be capacitively isolated and separated from the secondary, while to further reduce the capacitive effects both the primary and secondary of the transformer could be physically separated with respect to the core. It is therefore not intended to restrict the present invention to any specific construction except as defined in the appended claims.

What is claimed is:

l. In a low capacitance A.C. operated direct-current power supply a power transformer comprising a magnetic core having a throat portion, a primary winding Wound directly on said core for maximum magnetic coupling, a secondary winding, means for capacitively isolating the major portion of said secondary winding from said primary winding and said core comprising an insulated spool on which said secondary is wound, a portionvof said spool being threaded through said throat portion, the geometrical relation between said spool and said core being chosen such that, for a given core size, the capacitance between the secondary winding and primary winding, core and ground respectively are kept to a minimum value.

2. In the invention of claim 1, a secondary winding having at least two coils.

7 8 3. In the invention 0f claim 1. a Secondary winding 5. In the invention of daim 4, a secondary windin having a'center tap. having at least two coils.

4. In a low capacitance ALC. operated direct-current 6. In the invention of claim 4, a Secondary winding power supply a power transformer comprising i' 1 maghaving a center tap. Y netic core having an Iopen throat portion, a primary 5 winding consisting of tWo series-connected sections of References Cited in the file of this patent coil wound directly on said core for maximum magnetic UNITED STATES PATENTS coupling, a secondary windmg, means for capacitively isolating the major portion of said secondary winding 2,297,605 Camilli Sept. 29, 1942 from said primary winding and said core comprising an 10 2,479,656 Wiegand Aug. 23, 1949 insulated spool on which said secondary is wound, a 2,880,401 Mercier Mar. 3l, 1959 portion of said spool being threaded through said open throat portion, the geometrical relation between said OTHER REFERENCES spool and said core being chosen such that, for a given Low Capacity Transformers, by C. B. Adelmann, core size, the capacitance between the secondary wind- 15 C.Q. Radio Amateurs Journal, vol. 12, #7, pp. 40, 41, ing Aand the primary winding, core and ground respec- 42, July 1956.

tively are kept toa minimum value. 

